It is previously well known to use, for the control of industrial robots, sensors mounted on the robot hand. One example of such a field of application is automatic welding by means of an industrial robot, in which the sensor can be utilized both for detecting a weld joint and for automatically tracking the joint during the welding operation. Examples of such robot installations are described in prior US patent application Ser. No. 597,298 (filed on Apr. 6, 1984, now abandoned, in the names of Edling et al.) British application Nos. 2 088 095 and 2 110 427, and U.S. Pat. No. 4,306,144. Examples of sensors which are suitable for use in such installations are additionally described in U.S. Pat. Nos. 4,417,127 and 4,501,950.
A common problem with robot installations of the kind described above is that the coordinates of the sensor must be related to the coordinate system of the robot. In prior art installations it has been found that an individual calibration of each sensor must be carried out after its mounting on the robot. The reason for this is that inaccuracies in the attachment of the sensor and unavoidable variations in dimensions and in measuring systems between different examples of the same sensor type have made it impossible to attain the often very high requirements for accuracy (typically of the order of magnitude of .+-.0.1 mm) in the positioning of the work tools of the robot without the calibration mentioned. A manual calibration of a hitherto known kind is very time-consuming and entails, for example during replacement of a defective sensor, a considerable loss of production.
It has been proposed to design the sensor and its attachment in such a way as to obtain a mechanically very well-defined sensor mounting. However, such an embodiment would require a very high accuracy of the sensor attachment and of the mechanical construction of the sensor and of its measuring system. However, even with a high accuracy in these respects, it would probably be impossible to fulfil the demands which are often placed on an accurate positioning of the robot tool. Furthermore, for practical reasons, only one way or a few ways of mounting the sensor on the robot could be used in each particular case. This is a considerable disadvantage, since it is often very desirable to be able to mount the sensor in an arbitrary way in relation to the tool in view of the demands for accessibility which may be placed in each particular case.
From published European patent application Nos. 42 960 and 114 505, and from published International patent application WO85/03368, various methods are known for determining the position of the hand of an industrial robot independently of the robot's positioning system, and for calibration of the robot's positioning system. There are no suggestions in these publications of the particular problems of calibrating a sensor mounted on a robot hand, i.e. of determining the relation between the coordinate system of the robot and the coordinate system of such a sensor.